This invention pertains to a silicon nitride (Si.sub.3 N.sub.4) ceramic body and a process for preparing the same.
Silicon nitride ceramics are recognized for their excellent mechanical and physical properties, including good wear resistance, low coefficient of thermal expansion, good thermal shock resistance, high creep resistance and high electrical resistivity. In addition, silicon nitride ceramics are resistant to chemical attack, particularly to oxidation. Because of these attributes, silicon nitride is useful in a variety of wear and high temperature applications, such as cutting tools and parts in pumps and engines.
Typically, the densification of silicon nitride requires the presence of densification aids, such as MgO, Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, CeO.sub.2, SiO.sub.2, and ZrO.sub.2. A powder mixture is usually prepared comprising silicon nitride and one or more of such densification aids and heated under conditions described hereinafter. The densification aids form a liquid phase into which the silicon nitride is dissolved. Thereafter the dissolved silicon nitride coalesces to form a densified silicon nitride body.
Typically, the densification of the aforementioned powder mixture is carried out by one of four general methods: hot pressing (HP), hot isostatic pressing (HIP), pressureless sintering, or low pressure gas sintering. Hot pressing involves the simultaneous application of heat and mechanical pressure to the powder mixture at temperatures high enough to cause densification. Typical hot pressing conditions include a nitrogen atmosphere, a temperature in the range from about 1650.degree. C. to about 1900.degree. C., and a pressure in the range from about 2000 psig to about 5,000 psig. The pressure is usually applied to the powder mixture by means of a uniaxial ram press.
In the hot isostatic pressing method, the powder mixture is placed in a non-permeable, deformable container to which heat and pressure are applied. In this method pressure in the range from about 10,000 psig to about 30,000 psig is applied equally to all faces of the powder compact, usually by means of a pressurized gas. The temperature of this method typically ranges from about 1800.degree. C. to about 2100.degree. C.
Pressureless sintering generally connotes a process of thermally densifying pre-pressed powder compacts without the use of a container for the compact and without the external application of high pressure to the compact. Thus, the powder mixture is pre-pressed into the desired near net shape and then heated to a high temperature, typically in the range from about 1650.degree. C. to about 1800.degree. C. under a flow of inert gas, such as nitrogen, at one atmosphere pressure. Low pressure gas sintering is similar to pressureless sintering with the exception that low over-pressures of the inert gas up to about 150 psig are applied.
Densification of silicon nitride alone normally does not go to completion in the absence of high pressure. For example, the density of the silicon nitride ceramic body might only reach 80 or 90 percent of its theoretical value, whereas a density of 98 percent or more is required in order to achieve a ceramic having excellent mechanical and physical properties, such as high fracture strength and high fracture toughness. In addition, at high temperatures and low pressures silicon nitride decomposes into elemental silicon and nitrogen. Thus, the commercial need for fully densified silicon nitride ceramics having excellent fracture strength and fracture toughness is currently met predominantly by hot pressing or hot isostatic pressing silicon nitride with densification aids.
Disadvantageously, however, the hot pressing and hot isostatic pressing methods require complicated high pressure equipment. Moreover, only a ceramic having a simple shape can be prepared, which must be thereafter diamond ground into a more complicated net shape.
It is known in the art that the sinterability of silicon nitride-based systems is highly sensitive to composition. For example, the composition of the glassy phase critically affects the final sintered density. In addition, the solubility of silicon nitride in the liquid phase and the amount and wettability of the liquid phase all affect the homogeneity and morphology of the finished ceramic, which in turn affect its physical and mechanical properties. As a result it is difficult to prepare by pressureless or low pressure gas sintering a fully densified silicon nitride ceramic body with properties, such as, fracture toughness and fracture strength which meet current commercial needs.
It would be desirable to prepare a fully densified silicon nitride ceramic body by pressureless or low pressure sintering. Such a process would reduce the need for high pressure equipment, eliminate the need for diamond grinding, and readily provide complicated near net shapes in large numbers. In addition, it would be even more desirable if such pressureless or low pressure methods of densification also provided a silicon nitride ceramic body having excellent physical properties, such as, high fracture strength and high fracture toughness.